If rain is sneaking under your exterior or French doors and damaging your floors, the culprit is almost always a faulty threshold. This is a surprisingly common problem, and with the right approach, it's entirely fixable, even if your original installation was a little unconventional. In House Doctor, Ron shows us a step-by-step guide to diagnosing and replacing a leaking exterior door threshold.
Diagnosing the Problem
The first step is figuring out exactly how water is getting in. In most cases, the threshold's high ridge should align precisely with the bottom of the door. When it doesn't — even by as little as an inch — water running down the face of the door will skip over the drip cap and flow straight back inside, especially when there's wind driving the rain. If you can see that the vinyl seal at the bottom of the door is barely making contact with the threshold, that's a sure sign a replacement is needed. For more on achieving a draft-free entry, check out our guide on how to install weather stripping on an entry door.
Getting a Custom Threshold Made
Off-the-shelf thresholds don't always come in the right size or profile for every door. If that's the case, don't be discouraged. Take careful measurements and sketch out the exact dimensions you need, then bring that drawing to a local millwork or cabinet shop. They can fabricate a custom length of threshold to your specifications — well worth the extra step for a proper, lasting fix.
Removing the Old Threshold
Removal can be straightforward or tricky depending on how the original threshold was installed. In some cases, the door frame (or jamb) was installed on top of the threshold, trapping it underneath. If that's your situation, you won't be able to simply pry it out. Instead, cut through the center of the threshold with a saw, then work each piece free from the sides. Watch for screws or nails at the ends.
Preparing the Jamb
Once the old threshold is out, inspect the bottom of the door jamb. If there's an undercut or gap where the old threshold used to slide underneath, you'll need to fill it in before the new threshold can be dropped in from above. A polyester resin wood filler mixed with hardener works well for this. This is the same professional-grade material used to repair dry rot in window frames and shutters. Apply it to the gap, allow it to firm up to a rubbery consistency (usually around five minutes), then shape it with a knife before it hardens completely. This restores the jamb so it runs flush to the floor, giving the new threshold a clean surface to meet.
Leveling the Subfloor
If the tile or flooring on one side of the threshold is higher than the subfloor on the other side, you'll need to shim up the subfloor area so everything sits level. Cut a piece of quarter-inch plywood to fit the opening. If it's still slightly shy of the right height, add layers of roofing paper as shims until the plywood sits perfectly flush with the surrounding floor. Once level, nail it down with ring-shank galvanized nails spaced about eight to ten inches apart.
Installing the New Threshold
With the subfloor level and prepped, you're ready to set the new threshold. Apply a generous bead of subfloor adhesive to the plywood beneath, then run a bead of silicone caulk along the front edge and the back edge where it meets the tile. This prevents water from sneaking in from either direction. Notch the ends of the threshold (sometimes called a coping cut) so it fits snugly into the jambs on each side — expect a bit of trial and error to get a tight fit. Press the threshold firmly into place, then secure it with galvanized nails countersunk just below the surface. Fill the nail holes with wood filler once it's set.
Installing New Door Bottoms
Replacing the door bottoms is an important finishing step for two reasons. First, a good door bottom includes a rain diverter that channels water away from the edge of the door and out onto the tapered threshold, directing it toward the outside rather than into the house. Second, the vinyl gasket or seal on the door bottom will make firm contact with the high ridge of the new threshold, creating a watertight seal.
To get the right fit, you'll need approximately a half-inch gap between the bottom of the door and the top of the threshold. Use a compass set to a half-inch to scribe a line across the door, then trim to that line with a saw and clean up the edge with a power plane. Before installing the door bottom, paint the freshly cut wood to seal it against moisture absorption. Attach the vinyl weatherstripping door bottom with stainless steel screws.
If the damage to your threshold has spread to the frame itself, you may need to consider installing a new exterior door entirely
Testing and Maintenance
Once everything is in place, do a water test — run a hose across the door and check inside for any leaks. If the seal is good, you should find it completely dry.
When the threshold dries fully, sand it lightly and apply three coats of spar varnish, ideally one with an ultraviolet filter (available at marine supply stores). After that, a light scuff sanding and a fresh coat of varnish once a year is all it takes to keep it in good shape. Touch up any areas on the jamb where filler was applied, and your door should be watertight for years to come.
